The present invention relates to a CO gas sensor. The CO gas sensor of the present invention is suitable for detecting the concentration of a tiny amount of CO gas in an atmosphere rich in hydrogen gas and is suitably used in a fuel cell which employs a methanol-reforming gas as a fuel gas.
A novel gas sensor disclosed in the International Publication No. WO97/40371. The principle of the pulse method used in this gas sensor will be explained with reference to FIGS. 1-3. As shown in FIG. 1, this CO gas sensor has an electrolyte membrane 3 between a detection electrode 1 and a counter electrode 2. Positive voltage is applied to the detection electrode 1 by a sensor controlling unit 5. As shown in the upper part of FIG. 2, when voltage applied to the detection electrode is varied from relatively high CO oxidation potential to relatively low CO adsorption potential, a transient current (response current) flows as shown in the lower part of the same figure. This response current decreases with an increase in concentration of CO in a test gas. Then, the concentration of CO can be determined by reference to a calibration curve for the relationship between the rate of decrease of current and the CO concentration and comparing the measured rate of decrease in current with the calibration curve as shown in FIG. 3.
The present inventors have noted the following problems in the above gas sensor:
When the response current flowing between a detection electrode and a counter electrode is large, the performance of the sensor is reduced.
The sensitivity to CO gas is in need of improvement.
Insensitivity to environmental factors such as temperature and humidity is desirable; that is, a CO sensor which can measure over a wide temperature range and humidity range is needed.
A reference electrode has been used in the prior art CO sensor. However, from the viewpoint of miniaturization and weight reduction by decrease in the number of parts and, therefore, reduction in the manufacturing cost, it is desirable to omit this reference electrode.
In order to solve one or more of the aforementioned problems, the present invention provides a CO gas sensor including a detecting unit in which a solid electrolyte membrane is held between a detection electrode and a counter electrode and a voltage source which applies voltage between the detection electrode and the counter electrode and which varies the voltage. The detection electrode includes a first electrochemically active catalyst carried on an electrically conductive porous body and a reaction layer formed thereon having a density of 1 ng/cm2-100 xcexcg/cm2 and a thickness of 0.3 nm-15 xcexcm. The counter electrode includes a second electrochemically active catalyst carried on an electrically conducive porous body. In the CO gas sensor thus constructed, the current flowing between the detection electrode and the counter electrode can be minimized. Accordingly, elevated temperature in the sensor and drying of the electrolyte membrane can be prevented, leading to a long life for the sensor. Since the current can be small, the sensitivity to CO gas is improved.
In addition, the temperature range and humidity range over which the CO gas sensor can be effectively operated are widened.
Further, stable measurement of CO gas becomes possible even without a reference electrode.